1. Field of the Invention
The present invention relates to polyester laminates having a resin layer containing a polyester resin which is composed of diol units having a cyclic acetal structure and dicarboxylic acid units.
2. Description of the Prior Art
Laminates having a surface layer made of a heat-sealable resin have been widely used for the production of packaging materials and containers. Typical examples of such laminates include those made of polyolefin, particularly, polyethylene. Generally, polyethylene is poor in the gas barrier properties as well as in the preservation of fragrance of contents because it adsorbs fragrance of contents and allows low-boiling components to migrate. In addition, nutritive components of food such as vitamins are adsorbed by polyethylene. Therefore, polyethylene is not suitable in the application fields such as food packaging materials and containers which require gas barrier properties, fragrance preservation and non-adsorptivity of nutritive components.
In place of polyethylene, poly(ethylene terephthalate) excellent in the gas barrier properties and fragrance preservation has been examined for its potential as the material for laminates. However, poly(ethylene terephthalate) is poor in the heat sealability and is not suited for processing by extrusion lamination because of its low melt strength. To solve this drawback, the use of a copolymerized poly(ethylene terephthalate) or a polyester composition has been proposed (JP 7-196899A, JP 2002-225211A, JP 2003-311888A). However, the proposed copolyester is not sufficient in the melt strength although having a good heat sealability, to make a stable production of laminates difficult. In addition, since the glass transition temperature is low, the migration of low-boiling components occurs upon being brought into contact with hot products to be stored and the fragrance preservation is also poor.